School of Physics - Theses
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ItemSpin-down signatures of young neutron stars( 2022)The spin down of neutron stars has been invoked to explain a wide variety of electromagnetic and gravitational-wave signals. This thesis explores two different signals associated with the spin down of neutron stars, one electromagnetic signal and one gravitational-wave signal. Binary neutron star coalescences, confirmed as the progenitor of at least some short Gamma-ray bursts (sGRBs) in 2017, are predicted to form either a black hole or a highly magnetized neutron star. Up to 20% of sGRBs observed by the Neil Gehrels Swift telescope display prolonged X-ray emission, sometimes called a ``canonical'' afterglow, consisting of three phases: an initial power-law luminosity decay; a 'plateau', lasting between 10 s and 105 s, during which the X-ray luminosity is approximately constant; and a final power-law decay. Previous authors have noted that the evolution of the canonical light curve is broadly consistent with the expected spin-down luminosity of a neutron star. Key ideas from analytic, one-zone models of plerions (also called pulsar wind nebulae) can be used to model the evolution of a synchtrotron nebula fuelled by the the spin-down luminosity of a neutron star formed in an sGRB. An analytic expression for time-dependent, spatially-averaged electron energy distribution in the nebula is found and used to calculate the light curve and the point-in-time spectra. The light curves predicted by the plerionic model are consistent with the shape and luminosity of the X-ray light curves and reproduce the observed correlation between plateau duration and luminosity (i.e. brighter plateaux end sooner). Furthermore, Bayesian parameter estimation comparing the point-in-time spectra to time-averaged spectra of six Swift sGRBs with canonical X-ray afterglows and of known redshift allows estimation of the parameters of the neutron-star central engine, including its poloidal field strength Bp and its rotation period P0 at birth, and injection parameters within the shock, including the energy range of the relativistic electrons and their power-law index. All six sGRBs favour a neutron star with Bp ~ 1011 T and P0 ~ s, consistent with the prediction the neutron star should be highly magnetized and rapidly spinning. We also apply the point-in-time spectra to four time-averaged spectra taken at four separate epochs in the X-ray afterglow of GRB130603B and infer the evolution of the magnetic field in the synchrotron bubble B. We find the evolution of B is slower than the expected evolution of the far-field limit of the stellar magnetic field. Rotating, non-axisymmetric neutron stars spin down via the emission of continuous gravitational waves which may be detectable by current terrestrial interferometers such as the advanced Laser Interferometric Gravitational-wave Observatory (LIGO) and advanced Virgo. Young core-collapse supernova remnants are likely hosts of young neutron stars and are common targets for wide-band directed searches for continuous gravitational waves targeting non-pulsating neutron stars. In this work, we present the results for two searches for continuous waves from neutron stars in young supernova remnants using a hidden Markov model (HMM). The HMM tracking scheme models the frequency evolution as a random walk with secular spin down and remains sensitive in the presence of stochastic spin wandering similar to that observed in pulsar timing observations. A search targeting twelve neutron stars in young supernova remnants in the second observing run (O2) of advanced LIGO using an HMM tracking scheme identifies 1012 potential candidates, 18 of which survive a series of standard vetoes. Further assessment of the 18 survivors based on their dependence on sky position and Doppler modulation confirms they are all consistent with terrestrial noise. A second search, conducted with the the LIGO-Virgo-KAGRA (LVK) collaboration, targets fifteen neutron stars in young supernova remnants in the first half of the third observing run (O3a) of advanced LIGO and advanced Virgo using three search pipelines, including an HMM tracking scheme, and reports no candidates consistent with an astrophysical origin after a rigorous veto and follow-up process. The HMM tracking scheme sets the first 95% confidence limits on gravitational-wave strain, h095%, for these targets with a random-walk signal model, reaching a sensitivity of h095% = 2.64 x 10-25 at 172 Hz for G353.6-0.7. The constraints on h095% are converted to upper limits on neutron-star ellipticity below 10^-5 above 150 Hz and constrain the maximum amplitude of internal r-mode oscillations below 10^-3 above 150 Hz.
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ItemObservational methods towards constraining the chemical evolution of galaxies( 2020)Understanding the array of physical processes that have shaped galaxy assembly remains one of the most fundamental pursuits in astrophysics. Gas in galaxies is enriched with heavy elements via stellar nucleosynthesis, but chemical abundances (``metallicity'') are also shaped by galaxy-scale processes including gas accretion, feedback-driven outflows, radial gas flows, interactions, and mergers. Metallicity measurements therefore afford one of our most powerful observational probes of galaxy evolution. In this thesis I explore the performance of observational methods for constraining (i) gas-phase metallicity in galaxies, and (ii) host dark matter halo masses of galaxies; the latter of which is critical to the physics of gas flows due to its contribution to the gravitational potential well of galaxies. A particular focus is the improved understanding of systematic uncertainties near instrumental limits, which will be vital to maximise the impact of surveys conducted with future facilities. Galaxy clustering is an efficient approach for drawing statistical connections between galaxies and their host dark matter haloes, however traditional methods are challenging to apply at z > 2 where imaging survey volumes are limited. I instead apply a counts-in-cell approach to photometric z ~ 2 candidates from a random-pointing Hubble Space Telescope survey, showing mean counts of N > ~5 per field are capable of constraining the large scale galaxy bias. The James Webb Space Telescope will achieve comparable number counts out to z ~ 8, and thus a similar JWST survey could place novel constraints on the halo masses of galaxies in the epoch of reionization. Global metallicities in low-mass galaxies afford important constraints on the impact of feedback-driven outflows on galaxy evolution. However at high-z, obtaining the requisite emission line measurements is observationally challenging. I use Keck/MOSFIRE spectroscopy to explore prospects for extending z ~ 1 - 2 metallicity measurements to lower masses. I find the dominant source of uncertainty arises from reduced number of emission lines as opposed to lower signal-to-noise, even at the detection limit. JWST/NIRSpec will revolutionise high-z metallicity studies due to the large suites of emission lines it will be able to assemble. Electron temperatures (T_e) measured with auroral lines are an important baseline in metallicity studies. However the faintness of auroral lines has hitherto limited spatially resolved T_e studies. I report two separate studies based on mapping auroral lines in integral-field spectroscopy (IFS) of low-z galaxies. Measurements of auroral lines in the SAMI Galaxy Survey afford new insights into the effects of ionisation parameter variations on recovered metallicity gradients. Applying these principles to Keck/KCWI IFS data of an edge-on disk galaxy, I measure an extra-planar temperature gradient and present preliminary evidence for extra-planar metallicity variations.
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ItemThe host galaxies of high-redshift quasars( 2020)In the early Universe, we observe supermassive black holes with masses of up to a billion times the mass of the Sun, accreting at or even above the Eddington limit. These high-redshift quasars are some of the most luminous objects in the Universe, and raise many questions about the formation and growth of the first black holes. Investigating their host galaxies provides a useful probe for understanding these high-redshift quasars. In the local Universe, there are clear correlations between the mass of a supermassive black hole and the properties of its host galaxy, indicating a black hole--galaxy co-evolution. Exploring how these black hole--host relations evolve with redshift can give valuable insights into why these relations exist. Studying the host galaxies of high-redshift quasars thus provides vital insights into the early growth of supermassive black holes and the black hole--galaxy connection. In this thesis I use three techniques to study the host galaxies of high-redshift quasars: the Meraxes semi-analytic model, the BlueTides hydrodynamical simulation, and observations with the Hubble Space Telescope. Meraxes is a semi-analytic model designed to study galaxy formation and evolution at high redshift. Using this model, I study the sizes, angular momenta and morphologies of high-redshift galaxies. I also use Meraxes to study the evolution of black holes and their host galaxies from high redshift to the present day. The model predicts no significant evolution in the black hole--host mass relations out to high redshift, with the growth of galaxies and black holes tightly related even in the early Universe. I also examine the growth mechanisms of black holes in Meraxes, finding that the majority of black hole growth is caused by internal disc instabilities, and not by galaxy mergers. I then use the BlueTides cosmological hydrodynamical simulation to investigate the detailed properties of quasar host galaxies at z=7. I find that the hosts of quasars are generally highly star-forming and bulge dominated, and are significantly more compact than the typical high-redshift galaxy. Using BlueTides I make predictions for observations of quasars with the James Webb Space Telescope, finding that detecting quasar hosts at these redshifts may be possible, but will still be challenging with this groundbreaking instrument. Finally, I use observations from the Hubble Space Telescope to obtain deep upper limits on the rest-frame ultraviolet luminosities of six z~6 quasars. I also detect up to 9 potential companion galaxies surrounding these quasars, which may be interacting with their host galaxies. Observations with the upcoming James Webb Space Telescope are needed to detect quasar host galaxies in the rest-frame ultraviolet and optical for the first time.